Radio communication system, radio terminal, and radio communication method

ABSTRACT

A radio communication system ( 1 ) has: a radio terminal (UE); a radio base station (BS 1 ) which transmits radio signals (RS 1 ) to the radio terminal (UE) corresponding to the feedback from the radio terminal (UE); and a radio base station (BS 2 ) which transmits, corresponding to the feedback from the radio terminal (UE), radio signals (RS 2 ) to the radio terminal (UE) at the frequency and time identical to the frequency and time at which the radio signals (RS 1 ) are transmitted. The time interval at which the radio terminal (UE) performs the feedback to the radio base station (BS 1 ) is longer than the time interval at which the radio terminal (UE) performs the feedback to the radio base station (BS 2 ).

TECHNICAL FIELD

The present invention relates to a radio communication system to whichcoordinated base station MIMO communications are applied, and alsorelates to a radio terminal and a radio communication method.

BACKGROUND ART

MIMO (Multi-Input Multi-Output) communications have conventionally beenknown as a technique capable of improving the frequency usage efficiencyin a radio communication system. In the MIMO communications, multiplesignal sequences are transmitted at the same frequency and time whileeach of the transmission side and reception side of radio signals usesmultiple antennas.

In order to exert the performance of MIMO communications, transmissioncontrol needs to be adaptively performed in accordance with the state ofa radio channel. For this reason, closed-loop control is introduced intoa radio communication system into which MIMO communications areintroduced. To put it more specifically, the reception side of radiosignals generates channel information in accordance with characteristicsof a radio channel (hereinafter, referred to as “channelcharacteristics”) and feeds back the channel information to thetransmission side. Feedback of the channel information is required in aradio communication system employing an FDD scheme, in particular, inwhich there is no correlation in the channel characteristics between theuplink and downlink.

In recent years, coordinated base station MIMO communications (alsotermed as “coordinated multi-cell transmission/reception”) haveattracted attention as a technique developed from MIMO communications.In the coordinated base station MIMO communications, multiple radio basestations communicate with a single radio terminal at the same frequencyand time (refer to Patent Document 1, for example). Using antennas ofmultiple radio base stations, the coordinated base station MIMOcommunications can increase the number of antennas usable in MIMOcommunications, and thereby achieve an increase in the transmissionspeed and also an improvement in the reception quality as compared withconventional MIMO communications.

PRIOR ART DOCUMENT

Patent Document

PATENT DOCUMENT 1: Published Japanese Translation of PCT InternationalApplication No. 2008-523665

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

In the case of performing the closed-loop control in the coordinatedbase station MIMO communications, however, a radio terminal providesfeedback to all of the multiple radio base stations, so that theoverhead associated with the feedback increases as compared with theconventional MIMO communications. To put it more specifically, whenfeedback control channels are established between the radio terminal andthe respective multiple radio base stations, the amount of consumptionof radio resources increases in the uplink.

Instead, a feedback control channel may be established with only oneradio base station among the multiple radio base stations; and the oneradio base station may transfer the feedback to the other radio basestations through base-station communications. With this way, theoverhead associated with the feedback may be reduced. In the case wherethe one radio base station transfers the feedback to the other radiobase stations, however, the feedback may possibly not be transferred tothe other radio base stations on time due to the influence of transferdelay. Thus, the coordinated base station MIMO communications may notfunction normally.

In this respect, an object of the present invention is to provide aradio communication system, a radio terminal and a radio communicationmethod which can reduce the overhead associated with feedback whileallowing coordinated base station MIMO communications to functionnormally.

The present invention has the following features to solve the problemsdescribed above. First of all, a first feature of the present inventionis summarized as a radio communication system (radio communicationsystem 1) comprising: a radio terminal (radio terminal UE); a firstradio base station (radio base station BS1) configured to transmit afirst radio signal (radio signal RS1) to the radio terminal inaccordance with feedback from the radio terminal; and a second radiobase station (radio base station BS2) configured to transmit a secondradio signal (radio signal RS2) to the radio terminal in accordance withfeedback from the radio terminal, at the same frequency and time as thefirst radio signal, wherein a time interval at which the radio terminalprovides the feedback to the first radio base station is longer than atime interval at which the radio terminal provides the feedback to thesecond radio base station.

A second feature of the present invention is summarized in that, in thefirst feature of the present invention, the first radio base stationupdates a first transmission antenna weight (transmission antenna weight1), used to control at least one of a phase and an amplitude of thefirst radio signal, in accordance with the feedback from the radioterminal, the second radio base station updates a second transmissionantenna weight (transmission antenna weight 1), used to control at leastone of a phase and an amplitude of the second radio signal, inaccordance with the feedback from the radio terminal, and a timeinterval at which the first radio base station updates the firsttransmission antenna weight is longer than a time interval at which thesecond radio base station updates the second transmission antennaweight.

A third feature of the present invention is summarized in that, in thefirst feature of the present invention, during a period for whichfeedback to the first radio base station is omitted, the radio terminalprovides the second radio base station with feedback based on firstchannel information (channel information 1) in accordance with acharacteristic of a channel from the first radio base station to theradio terminal, and second channel information (channel information 2)in accordance with a characteristic of a channel from the second radiobase station to the radio terminal.

A fourth feature of the present invention is summarized in that, in thethird feature of the present invention, during the period for whichfeedback to the first radio base station is omitted, the radio terminalnormalizes the second channel information by using the first channelinformation as a reference and feeds back the normalized second channelinformation to the second radio base station.

A fifth feature of the present invention is summarized in that, in thethird feature of the present invention, during the period for whichfeedback to the first radio base station is omitted, the radio terminalfeeds back the first channel information and the second channelinformation to the second radio base station, and the second radio basestation normalizes the second channel information fed back from theradio terminal, while using the first channel information fed back fromthe radio terminal as a reference, and transmits the second radio signalin accordance with the normalized second channel information.

A sixth feature of the present invention is summarized in that, in thethird feature of the present invention, the first channel information isinformation indicating a characteristic of a channel between eachtransmission antenna of the first radio base station and a correspondingreception antenna of the radio terminal, and the second channelinformation is information indicating a characteristic of a channelbetween each transmission antenna of the second radio base station and acorresponding reception antenna of the radio terminal.

A seventh feature of the present invention is summarized in that, in thethird feature of the present invention, the first channel information isa first transmission antenna weight (transmission antenna weight 1) usedto control at least one of a phase and an amplitude of the first radiosignal, or an index indicating the first transmission antenna weight,and the second channel information is a second transmission antennaweight (transmission antenna weight 2) used to control at least one of aphase and an amplitude of the second radio signal, or an indexindicating the second transmission antenna weight.

An eighth feature of the present invention is summarized in that, in thefirst feature of the present invention, the first radio base station isa radio base station located farther from the radio terminal than thesecond radio base station.

A ninth feature of the present invention is summarized in that, in thefirst feature of the present invention, the first radio base station isa radio base station having higher processing performance than thesecond radio base station.

A tenth feature of the present invention is summarized in that, in thefirst feature of the present invention, the first radio base station isa radio base station having lower processing performance than the secondradio base station.

An eleventh feature of the present invention is summarized in that, inthe first feature of the present invention, the first radio base stationis a radio base station having a smaller fluctuation in a channel withthe radio terminal than the second radio base station.

A twelfth feature of the present invention is summarized in that, in thefirst feature of the present invention, the first radio base station isa radio base station having a larger loss in a channel with the radioterminal than the second radio base station.

A thirteenth feature of the present invention is summarized as a radioterminal (radio terminal UE) comprising: a receiver (receiver 121)configured to receive a first radio signal (radio signal RS1) from afirst radio base station (radio base station BS1) and to receive asecond radio signal (radio signal RS2) from a second radio base station(radio base station BS2), at the same frequency and time as the firstradio signal; and a transmitter (transmitter 122) configured to transmitfeedback to the first radio base station and the second radio basestation, wherein a time interval at which the transmitter provides thefeedback to the first radio base station is longer than a time intervalat which the transmitter provides the feedback to the second radio basestation.

A fourteenth feature of the present invention is summarized as a radiocommunication system comprising: a first radio base station (radio basestation BS1) configured to transmit a first radio signal (radio signalRS1) to a radio terminal (radio terminal UE) in accordance with feedbackfrom the radio terminal; and a second radio base station (radio basestation BS2) configured to transmit a second radio signal (radio signalRS2) to the radio terminal in accordance with feedback from the radioterminal, at the same frequency and time as the first radio signal,wherein one of the first radio base station and the second radio basestation instructs the radio terminal to set a time interval forproviding the feedback to the first radio base station to be longer thana time interval for providing the feedback to the second radio basestation.

A fifteenth feature of the present invention is summarized as a radiocommunication method comprising the steps of: transmitting a first radiosignal to a radio terminal (radio terminal UE) from a first radio basestation (radio base station BS1) in accordance with feedback from theradio terminal; and transmitting a second radio signal to the radioterminal from a second radio base station (radio base station BS2) inaccordance with feedback from the radio terminal, at the same frequencyand time as the first radio signal, wherein a time interval at which theradio terminal provides the feedback to the first radio base station islonger than a time interval at which the radio terminal provides thefeedback to the second radio base station.

According to the features of the present invention, it is possible toprovide a radio communication system, a radio terminal and a radiocommunication method which can reduce the overhead associated withfeedback while allowing coordinated base station MIMO communications tofunction normally.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configuration diagram of a radio communicationsystem according to a first embodiment and a second embodiment of thepresent invention.

FIG. 2 is a block diagram showing a configuration of a radio terminalaccording to the first embodiment of the present invention.

FIG. 3 is a block diagram showing a configuration of a radio basestation (first radio base station) according to the first embodiment andthe second embodiment of the present invention.

FIG. 4 is a block diagram showing a configuration of a radio basestation (second radio base station) according to the first embodiment ofthe present invention.

FIG. 5 is a diagram for describing normalization processing performed bya normalization unit according to the first embodiment of the presentinvention (part 1).

FIG. 6 is a diagram for describing normalization processing performed bya normalization unit according to the first embodiment of the presentinvention (part 2).

FIG. 7 is a flowchart showing an operation of the radio communicationsystem according to the first embodiment of the present invention.

FIG. 8 is a block diagram showing a configuration of a radio terminalaccording to a second embodiment of the present invention.

FIG. 9 is a block diagram showing a configuration of a radio basestation (second radio base station) according to the second embodimentof the present invention.

FIG. 10 is a flowchart showing an operation of the radio communicationsystem according to the second embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

Next, a description will be given of a first embodiment, a secondembodiment, and other embodiments of the present invention withreference to the drawings. In the following description of the drawingsin the embodiments, same or similar reference signs denote same orsimilar elements and portions.

First Embodiment

In the first embodiment, a description will be given of (1)Configuration of Radio Communication System, (2) NormalizationProcessing, (3) Base Station Selection Processing, (4) Operation ofRadio Communication System, and (5) Effects of First Embodiment.

(1) Configuration of Radio Communication System

To begin with, a description will be given of (1.1) Overall SchematicConfiguration, (1.2) Configuration of Radio Terminal UE, (1.3)Configuration of Radio Base Station BS1, and (1.4) Configuration ofRadio Base Station BS2.

(1.1) Overall Schematic Configuration

FIG. 1 is a schematic configuration diagram of a radio communicationsystem 1 into which coordinated base station MIMO communications(coordinated multi-cell transmission/reception or coordinatedmulti-point transmission/reception (CoMP)) are introduced.

The radio communication system 1 has a configuration based onLTE-Advanced, which is considered as the fourth generation (4G) mobilephone system, for example. The radio communication system 1 employs theFDD (Frequency Division Duplex) scheme as a duplex communication system.Hereinafter, a description will be mainly given of coordinated basestation MIMO communications in the downlink.

As shown in FIG. 1, the radio communication system 1 has a radio basestation BS1 (first radio base station), a radio base station BS2 (secondradio base station), a radio terminal UE and a control device 11. Theradio terminal UE is located in an overlapping portion of a cell C1formed by the radio base station BS1 and a cell C2 formed by the radiobase station BS2.

The radio base station BS1 and the radio base station BS2 may be amacrocell base station or a femtocell base station. The femtocell basestation is a small radio base station mainly installed indoors.

The radio base station BS1 and the radio base station BS2 are connectedto each other via a backhaul network 10 which is a wired communicationnetwork. The control device 11 provided in the backhaul network 10 andcontrols the radio base station BS1 and the radio base station BS2 viathe backhaul network 10. The radio base station BS1 and the radio basestation BS2, however, are capable of directly performing base-stationcommunications without using the control device 11.

Closed-loop control is introduced into the coordinated base station MIMOcommunications in the radio communication system 1.

The radio base station BS1 transmits a radio signal RS1 (first radiosignal) to the radio terminal UE in accordance with feedback from theradio terminal UE. The radio base station BS2 transmits a radio signalRS2 (second radio signal) to the radio terminal UE in accordance withfeedback from the radio terminal UE at the same frequency and time asthe radio signal RS1.

The radio base station BS1 performs weighting processing (called“pre-coding”) of a transmission signal in accordance with feedback fromthe radio terminal UE and then transmits the radio signal RS1 includingthe weighted transmission signal. With the weighting processing, thephase and the amplitude of the transmission signal (radio signal RS1)are controlled for each transmission antenna of the radio base stationBS1.

Likewise, the radio base station BS2 performs weighting processing of atransmission signal in accordance with feedback from the radio terminalUE and then transmits the radio signal RS2 corresponding to the weightedtransmission signal. With the weighting processing, the phase and theamplitude of the transmission signal (radio signal RS2) are controlledfor each transmission antenna of the radio base station BS2.

Here, a transmission signal sequence (also called a “stream”) includedin the radio signal RS1 transmitted by the radio base station BS1 and atransmission signal sequence included in the radio signal RS2transmitted by the radio base station BS2 may be the same or different.

When the transmission signal sequence included in the radio signal RS1and the transmission signal sequence included in the radio signal RS2are the same (during single stream transmission), the reception qualityis improved because of the diversity effect. As shown in FIG. 1, whenthe radio terminal UE is located at cell edges of the radio base stationBS1 and the radio base station BS2, the reception quality is preferablyimproved by single stream transmission.

When the transmission signal sequence included in the radio signal RS1and the transmission signal sequence included in the radio signal RS2are different (duringmulti-stream transmission), the transmission speedis improved. When the radio base station BS1 or the radio base stationBS2 is a femtocell base station, for example, and the radio terminal UEis located near the radio base station BS1 and the radio base stationBS2, the transmission speed is preferably improved by multi-streamtransmission.

The time intervals at each of which the radio terminal UE providesfeedback to the radio base station BS1 (hereinafter, feedback interval1) is longer than the time intervals at each of which the radio terminalUE provides feedback to the radio base station BS2 (hereinafter,feedback interval 2). The feedback interval 1 is an integral multiple ofthe feedback interval 2, for example.

In this embodiment, an assumption is made that the radio terminal UEprovides feedback to the radio base station BS2 in each frame while theradio terminal UE provides feedback to the radio base station BS1 at aratio of once in multiple frames, for example.

As described above, the radio terminal UE omits feedback to the radiobase station BS1 in an appropriate manner. Hereinafter, a descriptionwill be given of a case where feedback to the radio base station BS1 isomitted in an appropriate manner, but the radio base station targetedfor feedback omission in an appropriate manner is selectable. A methodfor selecting the radio base station targeted for feedback omission inan appropriate manner will be described later.

During the period in which the radio terminal UE omits feedback to theradio base station BS1 (hereinafter, feedback omission period), theradio terminal UE provides the radio base station BS2 with feedbackbased on channel information 1 in accordance with the channelcharacteristics from the radio base station BS1 to the radio terminal UE(first channel information) and channel information 2 in accordance withthe channel characteristics from the radio base station BS2 to the radioterminal UE (second channel information). The channel characteristicsmean parameters such as the attenuation amount, phase rotation amount,delay amount and the like received by a radio signal when the radiosignal passes through the radio channel.

(1.2) Configuration of Radio Terminal UE

FIG. 2 is a block diagram showing a configuration of the radio terminalUE.

Note that, hereinafter, a description will be mainly given of a portionrelating to the present invention. Accordingly, it should be noted thatthere is a case where the radio terminal UE includes a component whichis not illustrated or the description thereof is omitted (such as apower supply unit).

As shown in FIG. 2, the radio terminal UE includes multiple antennas111, a transceiver 120, a controller 130 and a storage unit 140. In theexample shown in FIG. 2, the radio terminal UE includes the multipleantennas 111, but the radio terminal UE does not always need to includemultiple antennas, and may include only a single antenna 111.

The transceiver 120 is configured of a radio frequency (RF) circuit, abaseband (BB) circuit and the like, for example. The controller 130 isconfigured of a CPU, for example, and controls various functionsincluded in the radio terminal UE. The storage unit 140 is configured ofa memory, for example, and stores therein various types of informationused in control or the like of the radio terminal UE.

The transceiver 120 has a receiver 121 and a transmitter 122. Thecontroller 130 has a channel information generating unit 131 and anormalization unit 132.

The receiver 121 receives the radio signal RS1 from the radio basestation BS1 and also receives the radio signal RS2 from the radio basestation BS2. Note that, in a case of single stream transmission or thelike, the receiver 121 preferably receives the radio signal RS1 and theradio signal RS2 in the same phase.

The channel information generating unit 131 performs processing tocalculate a channel estimation value by using a reference signal (pilotsignal) or the like included in the radio signal RS1 received by thereceiver 121 (so called channel estimation). Here, the channelestimation value indicates the channel characteristics between each ofthe transmission antennas of the radio base station BS1 and acorresponding one of the reception antennas of the radio terminal UE(hereinafter, a channel estimation value 1).

The channel information generating unit 131 performs processing tocalculate a channel estimation value by using a reference signal (pilotsignal) or the like included in the radio signal RS2 received by thereceiver 121. Here, the channel estimation value indicates the channelcharacteristics between each of the transmission antennas of the radiobase station BS2 and a corresponding one of the reception antennas ofthe radio terminal UE (hereinafter, a channel estimation value 2).

The channel information generating unit 131 generates the channelinformation 1, which is information to be fed back to the radio basestation BS1, on the basis of the channel estimation value 1. The channelestimation value 1 does not always need to be the channel information 1,and the channel information 1 may be a transmission antenna weight 1,which is used in the weighting processing performed by the radio basestation BS1 (i.e., in control of at least one of the phase and theamplitude of the radio signal RS1), or an index indicating thetransmission antenna weight 1 (called “PMI (Pre-coding Matrix Index)”).

The channel information generating unit 131 generates the channelinformation 2, which is information to be fed back to the radio basestation BS2, on the basis of the channel estimation value 2. The channelestimation value 2 does not always need to be the channel information 2,and the channel information 2 may be a transmission antenna weight 2,which is used in the weighting processing performed by the radio basestation BS2 (i.e., in control of at least one of the phase and theamplitude of the radio signal RS2), or an index indicating thetransmission antenna weight 2.

The normalization unit 132 normalizes the channel information 2 whileusing the channel information 1 as a reference during the feedbackomission period. The normalization processing will be described later indetail.

The transmitter 122 transmits feedback to the radio base station BS1 atthe feedback intervals 1 and also transmits feedback to the radio basestation BS2 at the feedback intervals 2. In addition, the transmitter122 feeds back the channel information 2 normalized by the normalizationunit 132 to the radio base station BS2 during the feedback omissionperiod.

(1.3) Configuration of Radio Base Station BS1

FIG. 3 is a block diagram showing a configuration of the radio basestation BS1.

As shown in FIG. 3, the radio base station BS1 includes multipleantennas 211, a transceiver 220, a controller 230, a storage unit 240,and a wired communication unit 250. In the example shown in FIG. 3, theradio base station BS1 includes the multiple antennas 211, but the radiobase station BS1 does not always need to include multiple antennas, andmay include only a single antenna 211.

The transceiver 220 is configured of an RF circuit, a BB circuit and thelike, for example. The controller 230 is configured of a CPU, forexample, and controls various functions included in the radio basestation BS1. The storage unit 240 is configured of a memory, forexample, and stores therein various types of information used in controlor the like of the radio base station BS1. The wired communication unit250 communicates with the radio base station BS2 and the control device11 via the backhaul network 10.

The transceiver 220 has a receiver 221 and a transmitter 222. Thecontroller 230 has a weight controller 231.

When the radio terminal UE feeds back the channel information 1, thereceiver 221 receives the fed back channel information 1.

The weight controller 231 controls the transmission antenna weight 1used in transmission of the radio signal RS1. When the receiver 221receives the channel information 1, the weight controller 231 updatesthe transmission antenna weight 1 in accordance with the channelinformation 1 and notifies the transmitter 222 of the updatedtransmission antenna weight 1.

During the period in which the receiver 221 receives no channelinformation 1, i.e., during the feedback omission period, the weightcontroller 231 does not update the transmission antenna weight 1 andnotifies the transmitter 222 of the transmission antenna weight 1 thathas been used by the weight controller 231 until that point of time.

The transmitter 222 weights (pre-codes) a transmission signal by usingthe transmission antenna weight 1 notified from the weight controller231 and then transmits the radio signal RS1 including the weightedtransmission signal.

(1.4) Configuration of Radio Base Station BS2

FIG. 4 is a block diagram showing a configuration of the radio basestation BS2.

As shown in FIG. 4, the radio base station BS2 includes multipleantennas 311, a transceiver 320, a controller 330, a storage unit 340,and a wired communication unit 350. In the example shown in FIG. 4, theradio base station BS2 includes the multiple antennas 211, but the radiobase station BS2 does not always need to include multiple antennas, andmay include only a single antenna 211.

The transceiver 320 is configured of an RF circuit, a BB circuit and thelike, for example. The controller 330 is configured of a CPU, forexample, and controls various functions included in the radio basestation BS2. The storage unit 340 is configured of a memory, forexample, and stores therein various types of information used in controlor the like of the radio base station BS2. The wired communication unit350 communicates with the radio base station BS1 and the control device11 via the backhaul network 10.

The transceiver 320 has a receiver 321 and a transmitter 322. Thecontroller 330 has a weight controller 331.

The receiver 321 receives the channel information 2 fed back from theradio terminal UE. The channel information 2 received by the receiver321 may be normalized or not normalized.

The weight controller 331 controls the transmission antenna weight 2used in transmission of the radio signal RS2. The weight controller 331updates the transmission antenna weight 2 in accordance with the channelinformation 2 received by the receiver 321 and notifies the transmitter322 of the updated transmission antenna weight 2.

The transmitter 322 weights (pre-codes) a transmission signal by usingthe transmission antenna weight 2 notified from the weight controller331 and then transmits the radio signal RS2 including the weightedtransmission signal.

(2) Normalization Processing

Next, the normalization processing performed by the normalization unit132 will be described with reference to FIG. 5 and FIG. 6. Firstly, adescription will be given of a case where the channel information 1 isthe channel estimation value 1 and the channel information 2 is thechannel estimation value 2.

The channel estimation value has a value for each antenna (eachcombination of transmission and reception antennas), and even when thevalue itself is changed, the value can be appropriately processed on thereception side (signal isolation or the like) as long as the relativerelation of the values is maintained. For example, the channelestimation values 1 (a, b) and the channel estimation values 2 (c, d)shown in FIG. 5( a) are converted by multiplying all of the channelestimation values by a constant e (a×e, b×e, c×e, d×e) as shown in FIG.5( b). Even in this case, the relation between the phases of the channelestimation values 1 and the channel estimation values 2 is maintained,and the same phase relation is kept. Thus, there is no problem inparticular in this case.

In this respect, as shown in FIG. 6( a), the channel estimation value 1(a) and the channel estimation values 2 (b, c, d) each of which needs tobe fed back are converted by dividing the channel estimation values 2 bythe channel estimation value 1 as shown in FIG. 6( b). To put it morespecifically, the normalization through division resulting in the pathestimation value 1 (a/a)=(1) and the channel estimation values 2 (b /a,c /a, d/a) makes it possible to virtually always fix the channelestimation value 1 to be (1), and to eliminate a need for feedback tothe radio base station BS1.

In this example, the normalization is performed by dividing the channelestimation values 2 by the channel estimation value 1, but othernormalization methods may be used as long as the normalization isperformed by a calculation method that keeps the relative relationbetween the channel estimation value 1 and the channel estimation values2. Thus, the normalization may be performed by subtracting the channelestimation value 1 from each of the channel estimation values 2, forexample.

Meanwhile, assume a case where the channel information 1 is thetransmission antenna weight 1 or its index and the channel information 2is the transmission antenna weight 2 or its index. In this case, inaddition to the aforementioned processing, a transmission antenna weight2 corresponding to the normalized channel estimation value 2 is selectedfrom candidates of the transmission antenna weight 2 (called code book),for example, and then, the selected transmission antenna weight 2 or itsindex is fed back to the radio base station BS2.

Alternatively, the following method may be employed as a method forselecting an optimum PMI from a list (code book) of the indices oftransmission antenna weights 2 (termed as PMIs, herein). To put it morespecifically, a reception SNR using a PMI in the list is calculated foreach PMI on the basis of the channel estimation value 1 and the channelestimation values 2, and a PMI corresponding to the largest receptionSNR among the calculated reception SNRs is selected as the optimum PMI.If there are few SNRs in the list, such a method is effective. When thismethod is employed, the normalization unit 132 selects (searches for) aPMI to be fed back to the radio base station BS2. Here, the PMI allowsthe reception SNR to be the largest under a condition that the radiobase station BS1 is assumed to perform fixed transmission, on the basisof the channel estimation value 1 and the channel estimation values 2.

Note that, the radio base station BS1 uses a single transmission antennain the aforementioned example, but the same processing can be appliedeven when the number of transmission antennas of the radio base stationBS1 is two or more.

(3) Base Station Selection Processing

Next, a description will be given of base station selection processingto select a radio base station (radio base station BS1) targeted forfeedback omission in an appropriate manner. The base station selectionprocessing may be performed by any one of the radio terminal UE, theradio base station BS1, the radio base station BS2 and the controldevice 11.

As the base station selection processing, any of Selection Methods 1 to5 below is usable, for example. Here, it is not necessary to fixedly useany of Selection Methods 1 to 5. Selection Methods 1 to 5 may beappropriately switched from one to another and used in accordance withthe conditions of the radio terminal UE, the radio base station BS1 andthe radio base station BS2.

(Selection Method 1)

In Selection Method 1, a radio base station located far from the radioterminal UE is selected as the radio base station BS1 and a radio basestation located near the radio terminal UE is selected as the radio basestation BS2. Thus, the radio terminal UE always provides feedback to theradio base station BS2 located near the radio terminal UE and omits, inan appropriate manner, feedback to the radio base station BS1 locatedfar from the radio terminal UE. Accordingly, the transmission power forfeedback can be reduced, and the interference amount can be reduced aswell. Selection Method 1 is preferred if the remaining battery level ofthe radio terminal UE is low or the like. Note that, although SelectionMethod 1 needs information on the distance between the radio terminal UEand the radio base station BS1 and information on the distance betweenthe radio terminal UE and the radio base station BS2, the information onthe distances can be obtained by using a GPS or the like provided in theradio terminal UE.

(Selection Method 2)

In Selection Method 2, a radio base station having high processingperformance (macrocell base station, to be more specific) is selected asthe radio base station BS1 and a radio base station having lowprocessing performance (femtocell base station, to be more specific) isselected as the radio base station BS2. A femtocell base stationnormally exists near the radio terminal UE. Thus, the radio terminal UEalways provides feedback to the radio base station BS2 located near theradio terminal UE (femtocell base station) and omits, in an appropriatemanner, feedback to the radio base station BS1 located far from theradio terminal UE (macrocell base station). Selection Method 2 iseffective when no GPS or the like is provided in the radio terminal UE,for example.

(Selection Method 3)

In Selection Method 3, a radio base station having low processingperformance (femtocell base station, to be more specific) is selected asthe radio base station BS1 and a radio base station having highprocessing performance (macrocell base station, to be more specific) isselected as the radio base station BS2. A high transmission speed can beobtained by causing the radio base station having low processingperformance to perform weight fixed constant transmission and causingthe radio base station having high processing performance to performmulti-stream transmission using multiple antennas. Thus, SelectionMethod 3 is effective when a high transmission speed is required, or theremaining battery level of the radio terminal UE is high.

(Selection Method 4)

In Selection Method 4, a radio base station having a small channelfluctuation with the radio terminal UE is selected as the radio basestation BS1 and a radio base station having a large channel fluctuationwith the radio terminal UE is selected as the radio base station BS2.This is because the channel fluctuation with the radio base station BS1is preferably small since the radio base station BS1 performs weightfixed constant transmission. Selection Method 4 is effective when thereis a large difference in the channel fluctuation between the radioterminal UE and each radio base station.

(Selection Method 5)

In Selection Method 5, a radio base station having a large channel losswith the radio terminal UE is selected as the radio base station BS1 anda radio base station having a small channel loss with the radio terminalUE is selected as the radio base station BS2. The channel loss ismeasured as a difference between the transmission power in the radiobase station and the reception power in the radio terminal UE, andreflects the distance between the terminal and the base station. Forthis reason, the radio terminal UE always provides feedback to the radiobase station BS2 located near the radio terminal UE and omits, in anappropriate manner, feedback to the radio base station BS1 located farfrom the radio terminal UE. The channel loss is used for anotherapplication such as transmission power control, and is easilyobtainable. Thus, the existing system configuration can be effectivelyutilized with Selection Method 5.

(4) Operation of Radio Communication System

Next, a description will be given of an operation of the radiocommunication system 1 according to the first embodiment with referenceto FIG. 7.

When coordinated base station MIMO communications start, in step S11, aradio base station BS1 targeted for feedback omission in an appropriatemanner is selected by the processing described in (3) Base StationSelection Processing above. When a device other than the radio terminalUE (radio base station BS1, radio base station BS2 or control device 11)selects the radio base station BS1 targeted for feedback omission in anappropriate manner, the device instructs the radio terminal UE to omitfeedback on the channel information 1 to the radio base station BS1 inan appropriate manner.

If feedback to the radio base station BS1 is to be provided at thistiming (step S12; YES), the transmitter 122 of the radio terminal UEfeeds back the channel information 1 to the radio base station BS1 andfeeds back the channel information 2 to the radio base station BS2 instep S13. The receiver 221 of the radio base station BS1 receives thechannel information 1. The receiver 321 of the radio base station BS2receives the channel information 2.

In step S14, the transmitter 222 of the radio base station BS1 transmitsthe radio signal RS1 to the radio terminal UE by using the transmissionantenna weight 1 updated by the weight controller 231 in accordance withthe channel information 1 received by the receiver 221. The transmitter322 of the radio base station BS2 transmits the radio signal RS2 to theradio terminal UE by using the transmission antenna weight 2 updated bythe weight controller 331 in accordance with the channel information 2received by the receiver 321.

Meanwhile, if feedback to the radio base station BS1 is to be omitted atthis timing (step S12; NO), the normalization unit 132 of the radioterminal UE normalizes the channel information 2 while using the channelinformation 1 as a reference in step S15 by the processing described in(2) Normalization Processing above.

In step S16, the transmitter 122 of the radio terminal UE omits feedbackon the channel information 1 to the radio base station BS1 but feedsback the channel information 2 normalized by the normalization unit 132to the radio base station BS2. The receiver 321 of the radio basestation BS2 receives the normalized channel information 2. The weightcontroller 331 of the radio base station BS2 notifies the transmitter322 of the transmission antenna weight 2 updated in accordance with thechannel information 2 received by the receiver 321.

In step S17, the transmitter 322 of the radio base station BS2 weights atransmission signal by using the updated transmission antenna weight 2notified by the weight controller 331, and transmits the radio signalRS2 including the weighted transmission signal. In addition, thetransmitter 222 of the radio base station BS1 weights a transmissionsignal by using the transmission antenna weight 1, which is not updated,and transmits the radio signal RS1 including the weighted transmissionsignal.

(5) Effects of First Embodiment

The radio terminal UE omits, in an appropriate manner, feedback to theradio base station BS1. Thus, the overhead associatedwith the feedbackcan be reduced. To put it differently, during the feedback omissionperiod, the radio terminal UE needs to establish a feedback controlchannel only with the radio base station BS2. Thus, the amount ofconsumption of radio resources can be reduced in the uplink.

In the first embodiment, the radio terminal UE normalizes the channelinformation 2 while using the channel information 1 as a reference andfeeds back the normalized channel information 2 to the radio basestation BS2 during the feedback omission period. Accordingly, the radiobase station BS2 transmits the radio signal RS2 in accordance with thechannel information 2 whose relative relation with the channelinformation 1 is maintained. Thus, it is possible to function thecoordinated base station MIMO communications normally even during thefeedback omission period.

Second Embodiment

While the radio terminal UE normalizes the channel information in thefirst embodiment, the radio base station BS2 normalizes the channelinformation in the second embodiment. Hereinafter, a description will begiven of (1) Configuration of Radio Communication System, (2) Operationof Radio Communication System and (3) Effects of Second Embodiment.Here, a description will be given of differences from the firstembodiment.

(1) Configuration of Radio Communication System

FIG. 8 is a block diagram showing a configuration of a radio terminal UEaccording to a second embodiment. FIG. 9 is a block diagram showing aconfiguration of a radio base station BS2 according to the secondembodiment.

As shown in FIG. 8, the radio terminal UE does not include thenormalization unit 132, which is described in the first embodiment.Meanwhile, as shown in FIG. 9, the radio base station BS2 includes anormalization unit 332 having the same functions as the normalizationunit 132, which is described in the first embodiment. The other parts ofthe configuration are the same as in the case of the first embodiment.

(2) Operation of Radio Communication System

Next, a description will be given of an operation of the radiocommunication system 1 according to the second embodiment with referenceto FIG. 10. Here, since the processing in steps S21 to S24 is the sameas in the case of the first embodiment, the processing in steps S25 toS27 will be described.

In step S25, the channel information generating unit 131 of the radioterminal UE generates the channel information 1 and the channelinformation 2. Then, the transmitter 122 of the radio terminal UE omitsfeedback on the channel information 1 to the radio base station BS1 butfeeds back the channel information 1 and the channel information 2,which are generated by the channel information generating unit 131, tothe radio base station BS2. The receiver 321 of the radio base stationBS2 receives the fed back channel information 1 and channel information2.

In step S26, the normalization unit 332 of the radio base station BS2normalizes the channel information 2 while using the channel information1 as a reference by the same processing as the processing described in(2) Normalization Processing in the first embodiment. The weightcontroller 331 of the radio base station BS2 notifies the transmitter322 of the transmission antenna weight 2 updated in accordance with thechannel information 2 normalized by the normalization unit 332.

In step S27, the transmitter 322 of the radio base station BS2 weights atransmission signal by using the updated transmission antenna weight 2notified by the weight controller 331, and transmits the radio signalRS2 including the weighted transmission signal. In addition, thetransmitter 222 of the radio base station BS1 weights a transmissionsignal by using the transmission antenna weight 1, which is not updated,and constantly transmits the radio signal RS1 including the weightedtransmission signal.

(3) Effects of Second Embodiment

In the second embodiment, the radio terminal UE omits feedback to theradio base station BS1 but feeds back the channel information 1 and thechannel information 2 to the radio base station BS2 during the feedbackomission period. In addition, the radio base station BS2 normalizes thechannel information 2, which is fed back from the radio terminal UE,while using the channel information 1 as a reference, which is also fedback from the radio terminal UE, and updates the transmission antennaweight 2 in accordance with the normalized channel information 2.

Thus, the radio terminal UE needs to establish a feedback controlchannel only with the radio base station BS2 during the feedbackomission period. Accordingly, the amount of consumption of radioresources can be reduced in the uplink. In addition, the radio basestation BS2 transmits the radio signal RS2 in accordance with thechannel information 2 whose relative relation with the channelinformation 1 is maintained. Thus, it is possible to function thecoordinated base station MIMO communications normally even during thefeedback omission period.

Other Embodiments

As described above, the details of the present invention have beendisclosed by using the embodiments of the present invention. However, itshould not be understood that the description and drawings whichconstitute part of this disclosure limit the present invention. Fromthis disclosure, various alternative embodiments, examples, andoperation techniques will be easily found by those skilled in the art.

In the aforementioned embodiments, the channel information to be fedback is the channel estimation value, the transmission antenna weight orits index, but the channel information is not limited to these valuesand may be a channel prediction value which is calculated on the basisof the channel estimation value and which shows a future channelcharacteristic.

In the aforementioned second embodiment, the channel information 1 andthe channel information 2 are fed back to the radio base station BS2.Thus, the radio base station targeted for feedback omission in anappropriate manner can be switched from the radio base station BS1 tothe radio base station BS2 after the channel information 1 and thechannel information 2 are transferred from the radio base station BS2 tothe radio base station BS1.

As described above, it should be understood that the present inventionincludes various embodiments which are not described herein.Accordingly, the technical scope of the present invention should bedetermined only by the matters to define the invention in the scope ofclaims regarded as appropriate based on the description.

Note that the entire content of Japanese Patent Application No.2009-128169 (filed on May 27, 2009) is incorporated herein by reference.

INDUSTRIAL APPLICABILITY

As described above, the radio communication system, the radio terminaland the radio communication method according to the present inventioncan reduce the overhead associatedwith feedback while allowingcoordinated base station MIMO communications to function normally. Thus,the radio communication system, the radio terminal and the radiocommunication method are useful in radio communications such as mobilecommunications.

1. A radio communication system comprising: a radio terminal; a firstradio base station configured to transmit a first radio signal to theradio terminal in accordance with feedback from the radio terminal; anda second radio base station configured to transmit a second radio signalto the radio terminal in accordance with feedback from the radioterminal, at the same frequency and time as the first radio signal,wherein a time interval at which the radio terminal provides thefeedback to the first radio base station is longer than a time intervalat which the radio terminal provides the feedback to the second radiobase station.
 2. The radiocommunication system according to claim 1,wherein the first radio base station updates a first transmissionantenna weight, used to control at least one of a phase and an amplitudeof the first radio signal, in accordance with the feedback from theradio terminal, the second radio base station updates a secondtransmission antenna weight, used to control at least one of a phase andan amplitude of the second radio signal, in accordance with the feedbackfrom the radio terminal, and a time interval at which the first radiobase station updates the first transmission antenna weight is longerthan a time interval at which the second radio base station updates thesecond transmission antenna weight.
 3. The radiocommunication systemaccording to claim 1, wherein during a period for which feedback to thefirst radio base station is omitted, the radio terminal provides thesecond radio base station with feedback based on first channelinformation in accordance with a characteristic of a channel from thefirst radio base station to the radio terminal, and second channelinformation in accordance with a characteristic of a channel from thesecond radio base station to the radio terminal.
 4. Theradiocommunication system according to claim 3, wherein during theperiod for which feedback to the first radio base station is omitted,the radio terminal normalizes the second channel information by usingthe first channel information as a reference and feeds back thenormalized second channel information to the second radio base station.5. The radiocommunication system according to claim 3, wherein duringthe period for which feedback to the first radio base station isomitted, the radio terminal feeds back the first channel information andthe second channel information to the second radio base station, and thesecond radio base station normalizes the second channel information fedback from the radio terminal, while using the first channel informationfed back from the radio terminal as a reference, and transmits thesecond radio signal in accordance with the normalized second channelinformation.
 6. The radiocommunication system according to claim 3,wherein the first channel information is information indicating acharacteristic of a channel between each transmission antenna of thefirst radio base station and a corresponding reception antenna of theradio terminal, and the second channel information is informationindicating a characteristic of a channel between each transmissionantenna of the second radio base station and a corresponding receptionantenna of the radio terminal.
 7. The radio communication systemaccording to claim 3, wherein the first channel information is a firsttransmission antenna weight used to control at least one of a phase andan amplitude of the first radio signal, or an index indicating the firsttransmission antenna weight, and the second channel information is asecond transmission antenna weight used to control at least one of aphase and an amplitude of the second radio signal, or an indexindicating the second transmission antenna weight.
 8. The radiocommunication system according to claim 1, wherein the first radio basestation is a radio base station located farther from the radio terminalthan the second radio base station.
 9. The radio communication systemaccording to claim 1, wherein the first radio base station is a radiobase station having higher processing performance than the second radiobase station.
 10. The radio communication system according to claim 1,wherein the first radio base station is a radio base station havinglower processing performance than the second radio base station.
 11. Theradio communication system according to claim 1, wherein the first radiobase station is a radio base station having a smaller fluctuation in achannel with the radio terminal than the second radio base station. 12.The radiocommunication system according to claim 1, wherein the firstradio base station is a radio base station having a larger loss in achannel with the radio terminal than the second radio base station. 13.A radio terminal comprising: a receiver configured to receive a firstradio signal from a first radio base station and to receive a secondradio signal from a second radio base station, at the same frequency andtime as the first radio signal; and a transmitter configured to transmitfeedback to the first radio base station and the second radio basestation, wherein a time interval at which the transmitter provides thefeedback to the first radio base station is longer than a time intervalat which the transmitter provides the feedback to the second radio basestation.
 14. A radio communication system comprising: a first radio basestation configured to transmit a first radio signal to a radio terminalin accordance with feedback from the radio terminal; and a second radiobase station configured to transmit a second radio signal to the radioterminal in accordance with feedback from the radio terminal, at thesame frequency and time as the first radio signal, wherein one of thefirst radio base station and the second radio base station instructs theradio terminal to set a time interval for providing the feedback to thefirst radio base station to be longer than a time interval for providingthe feedback to the second radio base station.
 15. A radio communicationmethod comprising the steps of: transmitting a first radio signal to aradio terminal from a first radio base station in accordance withfeedback from the radio terminal; and transmitting a second radio signalto the radio terminal from a second radio base station in accordancewith feedback from the radio terminal, at the same frequency and time asthe first radio signal, wherein a time interval at which the radioterminal provides the feedback to the first radio base station is longerthan a time interval at which the radio terminal provides the feedbackto the second radio base station.